Broadband antenna



Dec. 17, 1957 w, CLQPP r 2,817,084

BROADBAND ANTENNA Filed Aug. 5, 1953 g d I 6 I I gl I .iZzzz-Z AmINVENTOR. flaw. M621),

BY 77/41/11 flaw/17y,

71/01 rm/way United States atent O BROADBAND ANTENNA Roger W. Clapp,Manhattan Beach, and Thomas Hudspeth, Torrance, Calif., assignors, bymesne assignments, to Hughes Aircraft Company, a corporation of DelawareApplication August 5, 1953, Serial No. 372,571

6 Claims. (Cl. 343-770) This invention relates to antennas, and, moreparticularly, to a linear array of radiating elements for operation overa relatively wide band of frequencies.

As is well known, an antenna comprising a linear array of individualradiating elements is often used to form a beam of radiant energy whichis most narrow in a plane parallel to the axis of the array. The lengthof the array, that is, the aperture of the antenna, determines theminimum beam width obtainable whereas the spacing of the elements andtheir manner of excitation determines the magnitude and distribution ofsecondary beams or side lobes. In an antenna of this type it is requiredthat adjacent elements be spaced from each other a distance less thanone wavelength in order to form a single primary beam, with the resultthat mutual coupling between individual radiating elements may berelatively high. Consequently, the impedance of an element in the arrayis caused to difier substantially from the impedance of the same elementwhen operated by itself. In general, this effect presents littledifliculty when a large number of elements is used and the frequency ofoperation is substantially fixed. lf the antenna is to be operated atsubstantially difierent frequencies, however, the mutual coupling andtherefore the impedance of the radiating elements will changeappreciably from one frequency to the next. Thus, it is desirable tominimize mutual coupling in this case so that the antenna impedance willremain more nearly constant over the band of operating frequenciesthereby permitting a more perfect match to be obtained between theantenna and its associated transmission line.

Patent No. 2,615,132, entitled Directive Broad Band Slot Antenna System,issued October 21, 1952, on the application of Victor H. Rumsey (June 5,1946, Serial No. 674,437) discloses an antenna comprising a linear arrayof radiating elements operating in accordance with the slot principle.As pointed out in the patent, this type of antenna is particularlysuited for wide band operation because of the small amount of mutualcoupling present. To further reduce mutual coupling and also to provideindividual elements having superior impedance characteristics inthemselves, the present invention discloses an antenna comprising alinear array of improved slot-type radiators. Consequently, the antennaaccording to the present invention may be more closely matched to itsassociated transmission line over a broad band of operating frequencies.

It is an object of this invention, therefore, to provide a linear arrayof improved slot-type radiating elements.

It is another object to provide an antenna having a substantiallyconstant input impedance over a wide band of frequencies.

It is still another object to provide slot-type radiating elements whichare particularly adapted for operation over a wide band of frequencies.

It is a further object to provide a slot-type radiator configurationthat is less critical as to dimensional variations thereby facilitatingits manufacture.

2,817,084 Patented Dec. 17, 1957 ICE The novel features which arebelieved to be characteristic of this invention, both as to itsorganization and method of operation, together with further objects andadvantages, thereof, will be better understood from the followingdescription considered in connection with the accompanying drawing inwhich:

Fig. 1 is a perspective view of an antenna in accordance withtheinvention;

Fig. 2 is a schematic diagram of a transmission line system suitable forfeeding the antenna illustrated in Fig. 1.

With reference to Fig. 1, there is shown by way of illustration anantenna consisting of a three-element array of radiators 31, 32, 33 inaccordance with the present invention. As seen in Fig.1, each of theradiators 31, 32, 33 comprises a rectangular box or duct 10 having broadtop and bottom walls 11, 12, narrow side walls 13, 14, and a back wall15 closing one end of the duct 10. The spacing between side walls 13, 14is approximately .8 wavelength in free space while the spacing betweentop and bottom is approximately .3 wavelength in free space. Top wall 11is extended a short distance beyond side walls 13, 14 and twice foldedbackwardly as shown in Fig. 1. Similarly, bottom wall 12 is extendedbeyond walls 13, 14 a substantial distance and also ben-t back at anacute angle. By means of these bends, chokes 26 and 27 are formed alongthe top and bottom of each duct 10 to minimize radiation in a backwarddirection.

The open end of duct 10 provides a rectangular aperture which may beexcited with electromagnetic energy wherein the electric vector Eextends between the top and bottom walls 11, 12 parallel to the sidewalls 13, 14. To excite the aperture in this manner, a probe 16 isinserted into each of the ducts 10 through bottom wall 12 and disposedparallel to side walls 13, 14. Probe 16 is located at a point remotefrom the aperture and equidistant from side walls 13, 14. Connectedbetween the top and bottom walls 11, 12 of each duct 10 and disposedparallel to side walls 13 and 14 is a pair of conductive straps 17 and18 which provide inductive reactance. Straps 17 and 18, which are in theform of ribbon-like structures each having a width which issubstantially greater than its thickness, are positioned at the apertureof each duct 10 approximately one-fourth wavelength either side of aline 19 vertically bisecting the aperture, and have a width which isless than one-tenth wavelength in free space.

By placing each of the ducts 10 side by side, there is formed anexemplary three-element array according to this invention. As shown inFig. l, the top, bottom and back walls 11, 12 and 15, respectively, ofeach duct 10 may comprise single sheets of conductive material.Similarly, the adjacent sides of the ducts 10 may be formed with acommon conductive wall so that side walls 13, 14 associated withradiator 32 also serve as side walls 14, 13 associated with radiators31, 33, respectively.

Fig. 2 illustrates schematically a system of transmission lines suitablefor feeding the antenna described in Fig. 1. As seen in Fig. 2,individual radiators 31, 32, 33 are fed by branch transmission lines 21,22, 23, respectively, which terminate in the probes 16. Branchtransmission lines 21, 22, 23 are, in turn, connected to a maintransmission line 24 which is supplied with electromagnetic energy. Ifidentical transmission lines are used throughout, it will be necessaryto provide a matching section 25 at the junction of lines 21, 22, 23, 24for matching the impedance of the main transmission line 24 to theimpedance of the parallel combination of branch transmission lines 21,22, 23. As is apparent, this method of feeding the antenna isillustrative only, Since the feeding arrangement utilized will to someextent depend on the radiation pattern which is desired. That is to say,the

radiationpatternof theantenna as a whole depends upon the relativeamplitudes and phases of the energy contributed by the radiators which;in turn, may be controlled most conveniently by the design of the feedsystem in accordance with well knownprinciples.

'The'operati" not any one of the'radiators 31,32, 33' is as follows.'Straps 17 and '18 divide the apertureof the duct intolthre'e regions,-each of whichmay be regarded as aslo't'. Owing to -the one-halfwavelength separation of straps 17 and 18, the central region; that :is,the region bounded'by top wall 11, bottom" wall'12 and straps 17, 18 issubstantially resonant. Thus, probe 16'produces electroma netic waveswithin the duct 10 which radiate from the central region; The two outerregion's, bfi'the other hand, are highly inductive and do not radiate anappreciable amount of electromagnetic energy since the distances betweenstrap17 andflside'wall 13', and strap 18 and sidewall 14 aresubstantially less than one-half wavelength in free space. 'Byminimizing radiation from the sides of the aperture, the amount ofmutual coupling between adjacent radiators is correspondingly reduced,thereby improving the impedance characteristics of the antenna as awhole. In addition, straps 17, 18 also serve as inductances whichimprove the antenna impedance characteristics still further bycompensatingthe individual radiators 31, 32, 33. That is to say, straps17, 18 do not act as pure inductances, but rather provide an amount ofinductive reactance appropriate to compensate for the reactivecomponents of theindividual radiator impedances at all frequencieswithin a relatively broad band. Thus the straps perform two functionseach of which contributes to the desired result, namely, the provisionof an antenna having a substantially constant impedance over arelatively broad band of frequencies.

These facts are best evidenced by the voltage standing wave ratiomeasured at the branch transmission lines. In particular, an antennacomprising a three-element array of radiators in accordance with thisinvention, as illustrated in Fig. 1, and operated at frequencies in therange between 950 and 1150 megacycles produced a standing wave ratio oneach branch transmission line 21, 22, 23 which did not exceed 1.15. Itshould also be noted in this regard that the impedance characteristicsand also the radiation pattern of the antenna of this invention remainsubstantially the same even though the dimensions of the antennastructure are changed considerably. Thus the antenna may be readilyduplicated without the need for close manufacturing tolerances and theirattendant high cost.

Therefore what is claimed is:

1. In a box-type radiating element having a rectangular aperture formedin part by upper and lower elongated walls supported inspaced oppositionand in parallel planes and having a probe supported by one of said wallsfor exciting the aperture with electromagnetic waves, a pair ofsubstantially rectangular conductive straps each having a width which issubstantially greater than its thickness, said straps being connectedacross the upper and lower walls of the aperture with their widthsparallel to the vertical axis of the aperture and perpendicular to thedirection of elongation of the aperture, each of said straps beingspaced from the center of the aperture a distance of approximatelyone-quarter wavelength in free space to maintain the impedance of theradiating element substantially constant over a wide band of operatingfrequencies.

2. A radiator of electromagnetic waves comprising an apertured structureshaped substantially in the form of a rectangular duct having two broadwalls, two narrow walls, and a back wall closing one end of the duct,aprobe within said apertured structure for producing electromagneticwaves whereinthe electric vector extends between said broad wallsparallel to said narrow walls, said'probe being located adjacent saidback wall and equidistant from said narrow walls, means for couplingsaid probe to a source of electromagnetic energy, and a pair ofsubstantially rectangular conductive straps each having a widthwhichis-substantially greater than its thickness, said straps beingconnected across the mouth of said apertured structure, with theirwidths being parallel to the narrow walls of said structure, said strapsbeing equally spaced from the respective narrow walls and separatedacross the mouth of the aperture by'a distance of approximately one-halfwavelength in' free'space, thereby minimizing electric fields in thevicinity of said narrow walls and providing inductive reactance tocompensate for variations in the impedance of said apertured structurewith variations in the wavelength of saidelectromagnetic waves.

3. In an antenna of the type which comprises a series of rectangularducts, each closed at one end and open at the other end to form a seriesof rectangular apertures, and each including a probe for exciting theapertures with electromagnetic waves wherein the electric vector extendsbetween the broad walls parallel to the narrow walls of the ducts; apair of substantially rectangular conductive straps, the width of eachof said straps being substantially greater than its thickness, saidstraps being connected across each aperture with their widths parallelto the narrow walls of each duct, said straps being equally spaced fromthe respective narrow walls, and each of said straps within each ductfrom the other strap in that duct separated by a distance ofapproximately one-halr wavelength in free space, thereby minimizingelectric fields in the vicinity of the narrow walls and providinginductive reactance to compensate for variations in the impedance of theantenna with variations in the wavelength of the electromagnetic waves.

4. -An antenna comprising a series of rectangular ducts aligned so thatthe narrow walls of adjacent ones of said ducts are parallel andcontiguous, saidducts being closed at one endan'd openedat the other endto form a series of rectangular apertures, a probe within each duct forproducing electromagnetic waves wherein the electric vector extendsbetween the broad walls parallel to the narrow walls of said ducts, saidprobe being positioned at a point remote-from the open end andequidistant from the narrow walls of each of said ducts, means forcoupling the probes to a source of electromagnetic energy, and a pair ofsubstantially rectangular conductive straps, the width of each ofsaidstraps being substantially greater than its thickness, said straps beingconnected across the aperture with their widths parallel to the narrowwalls of each duct section,-said straps being equidistant from saidprobe and spaced from'each other a distance of approximately one-halfwavelength in free space, thereby minimizing electric fields in thevicinity of the narrow walls and providing inductive reactance tocompensate for variations .in the impedance of the antenna withvariations in the wavelength'of said electromagnetic waves.

5. A radiator of electromagnetic Waves comprising an apertured structureshaped substantially in the form of a rectangular duct-having two broadwalls, two narrow walls spaced from each other a distance in the rangebetween three-fourths and one wavelength in free space, and a back wallclosing one end of the duct, a probe within said apertured'structure forproducing electromagnetic waves wherein the electric vector extendsbetween said broad walls parallel to said narrow walls, said probe beinglocated adjacent said back wall and equidistant from said narrow walls,means for coupling said probe to a source of electromagnetic energy, anda pair of substantially rectangular conductive straps, the width of eachof said straps being substantially greater than its thickness, saidstraps being connected between said broad walls across the mouth of saidapertured structure with their widths parallel to the narrow wallsthereof, said straps being equally spaced from therespective narrowwalls and separated by a distance of approximately one-half wavelengthin free space, thereby minimizing electric fields in the vicinity ofsaid narrow walls and providing inductive reactance to compensate forvariations in the impedance of said apertured structure with variationsin the wavelength of said electromagnetic waves.

6. In an antenna of the type which comprises a series of rectangularducts, having transverse dimensions in the order of .3 and .8 wavelengthin free space, each closed at one end and open at the other end to forma series of rectangular apertures, and each including a probe forexciting the apertures with electromagnetic waves wherein the electricvector extends between the broad walls pararallel to the narrow walls ofthe ducts; a pair of substantially rectangular conductive straps, thewidth of each of said straps being substantially greater than itsthickness, said straps being connected across each aperture with theirwidths parallel to the narrow walls of each duct,

References Cited in the file of this patent UNITED STATES PATENTS2,283,935 King May 26, 1942 2,521,524 Kock Sept. 5, 1950 2,615,132Rumsey Octv 21, 1952

